In the plumbing field a nipple is a predetermined short length of pipe usually NPT(National Pipe Thread). A NPT threaded nipple can be made of almost any material and can be in sizes ranging from 1/16" to 24" NPS(Nominal Pipe Size). The plumbing nipple gets it's nickname because of it's similar function to that of a female nipple in humans and animals. It's usually a small protrusion which emanates fluid or gas.
"You want your nipple an 1 5/8" off of the finish wall in this situation."